Electronic equipment often uses cooling fans to maintain system temperature at proper levels. The speeds of fans can be controlled as a function of the equipment temperature or loading conditions. It is essential that fans operate reliably because the malfunction or breakdown of a fan could be destructive to electronic equipment that relies on the fan to prevent it from overheating.
One known method to control fan speed is by applying a pulse width modulation (PWM) to the input of the fan motor. With PWM systems, the duty-cycle, or on time, of the input voltage pulses can be varied to control the fan speed. One type of prior art circuit that uses PWM to control fan speed is shown in FIG. 1. Referring to FIG. 1, a signal from a PWM source is applied to a common-emitter type circuit that includes a resistive network R1, R3 and R4 and a transistor Q1. The output of this circuit is an amplified and inverted reproduction of the PWM input signal. When the PWM signal turns on transistor Q1, the fan motor is not driven because the transistor has a low resistance and most of the current flowing from the positive voltage source flows through pull-up resistor R5, through the transistor Q1 and goes to ground. On the other hand, when the PWM input signal is low enough that transistor Q1 is turned off, the current from the positive voltage source flows through resistor R5 and then through the output to drive the fan motor. One drawback associated with this type of prior art control circuit is that if the PWM input malfunctions and produces a steady high or above ground d.c. signal sufficient to turn transistor Q1 on, the circuit will produce a low or no voltage output signal that will, in turn, terminate operation of the fan. Accordingly, this type of control circuit does not reliably compensate for PWM signal failures, and can lead to fan shut down, overheating and possible damage to electronic equipment.
What is desired, therefore, is a PWM driven electronic control circuit for a fan or motor that reproduces the PWM signal during normal operation and that will compensate for a malfunction or complete failure of the PWM input signal and ensure the fan's continued operation.